Multiple reheating apparatus for steam turbines



April 7, 1970 HELLER T AL 3,504,495

MULTIPLE REHEATING APPARATUS FOR STEAM TURBINES Filed Oct. 30. 1967 2 Sheets-Sheet 1 Fig. i

April 7, 1970 HELLER ET AL 3,504,495

MULTIPLE REHEATING APPARATUS FOR STEAM TURBINES Filed Oct. 30. 1967 2 Sheets-Sheet 2 Fig. 2

3,504,495 Patented Apr. 7, 1970 3,504,495 MULTIPLE REHEATING APPARATUS FOR STEAM TURBINES Laszl Heller and Laszl Sziics, Budapest, Hungary, assignors to Transelektro Magyar Villamossagi Kulkereskedelmi, Budapest, Hungary Filed Oct. 30, 1967, Ser. No. 679,101 Claims priority, application Hungary, Nov. 4, 1966, HE487 Int. Cl. F01k 7/16, 7/32 U.S. Cl. 60-73 7 Claims ABSTRACT OF THE DISCLOSURE A steam turbine interstage steam reheating system in which heat released by the condensation of mercury vapour is used to reheat the interstage steam.

It is known that the thermal efficiency of a steam turbine cycle can be improved by raising the mean temperature at which heat is introduced to the cycle. One practical method of carrying this into effect is that the expansion of steam in a turbine is interrupted and the steam reheated between turbine stages. In principle, the thermal gain increases with the number of such reheats.

Known methods for carying out the steam reheating involve drawbacks, such as increased first cost due to the equipment required for passing the steam to a boiler, for reheating the steam and for then passing it back to the turbine. Further energy is lost due to the pressuure loss arising in the process. Another drawback of known designs is that the reheat piping necessarily contains a substantial quantity of steam, which leads to difficulties firstly in the case of quick shutdowns and also because of oscillations arising in the piping. The elimination of these diflficulties is possible by the provision of control and closing organs involving an additional pressure drop and an additional complication of equipment which might impair the safety of turbine control and the reliability of emergency shutdown arrangements.

In order to minimize the drawbacks mentioned it has been suggested that reheating be carried out by means of a transmitting medium. This would mean the steam taken out of the turbine between stages is re-superheated in a heat exchanger placed close to the turbine by means of a transmitting medium which is heated in a boiler. With a suitable choice of transmitting medium, it can be ensured that the required increase of boiler surface areas will not be too great, a transmitting medium having a much higher heat transfer rate than superheated steam being chosen. Transmitting media proposed heretofore would remain in a liquid state in the whole temperature range of the heat transmission in question.

A serious drawback of all forms of this second process put forward to date-especially in the case of a large number of reheat stages-is the rather large size of the heat exchangers required, which should be installed close to the turbine, involving layout problems difiicult to overcome, and substantial expense.

Due to the difiiculties and drawbacks mentioned not more than two reheat stages have normally been used in practice while reheating by transmitting media has not been in use to any extent.

The plant according to this invention is based on the perception that the use of an evaporating and condensing medium such as mercury (or a like liquid metal having similar physical properties) as the transmitting medium enables compact heat exchangers to be used that can be installed inside the steam duct connecting turbine stages or even inside the turbine casings, so that the space requirement of the turbine plant is not substantially increased. At the same time, due to the compactness of such heat exchangers, the quantity of steam stored in them is insignificant, so that no problems of control arise. Also, the circumstances mentioned make possible the application of a large number of reheat stages, due to which the application of mercury-or a similar, evaporating-condensing mediumas transmitting medium becomes extremely attractive from the thermal point of view. In the system according to the invention, mercury, or some other material having similar physical properties, used as a transmitting medium is evaporated in the boiler and then passed through a heat exchanger in heat transfer relation with interstage steam to effect reheating of the steam and condensation of the mercury or other liquid which is then returned to the boiler. The heat exchanger is thus a condenser, as against the heat exchangers of earlier proposals which were to be heated by a liquid transmitting medium (such as sodium, eutectic sodium-potassium, etc.).

The heat transfer rate of liquid media is worse than that of a condensing metal (especially if-as is the case with mercurythat metal is susceptible to drop condensation) and on the other hand the distribution of liquid media between several parallel tubes leads to grave difliculties because their viscosity is temperature dependent. Due to these reasons the effectiveness of steam-side fins in liquid-heated reheaters is counteracted, especially in the range of very compact constructions, by the poor heat transfer on the liquid side and the uneven distribution of the medium. Finally, the construction of suitably compact heat exchangers was rendered impossible by the fact that the liquid transmitting media considered are inflammable, enter into violent chemical reactions with water or, in the case of organic materials, they are inflammable, they are decomposed by heat and cause fouling, they need a blow-down, etc.

None of these dimculties arise to the same extent with condensing mercury, which thus enables more compact heat exchangers to be made for high ratings at low pressure drops.

(From the point of view of pump power requirement mercury is also more advantageous than other media, since it permits the application of natural circulation without pumping if suitable level differences are provided.)

In such compact heat exchangers there is preferably produced a laminar steam flow through very small gaps not exceeding 0.5 mm., resulting in an excellent heat transfer rate and very closely spaced steam-side fins, i.e. a high rate of finning. The combined effect of the excellent heat transfer rate and of the high rate of finning result in a heat transmission rate of the order of 10,000 kcal./ m. h. C. as related to the transmission-medium-side area. Such a value can only be attained by means of condensing mercury (or some other material having similar physical properties), since its heat transfer rate is of the order of a hundred thousand (as against a thousand or at most ten thousand for liquid transmtiting media).

The danger of the very small heat exchanger gaps getting clogged is small, since they carry pure medium (the steam of a high-pressure power station). Use of the present invention permits a turbine to approacheven with three or more reheat stagesisothermal expansion and through this to improve the thermal efficiency of power stations.

The invention will now be described with reference to the accompanying drawings in which:

FIGURE 1 is a steam diagram for a turbine having three steam reheat stages according to the invention and showing the approach to isothermal steam expansion; and

FIG. 2 is a diagram showing the system.

Referring to FIGURE 2, a steam turbine has four stages 2, the path of steam through the turbine being indicated by reference 4. In the steam ducting between each stage is a heat exchanger 5 having steam-side fins, each heat exchanger being adapted to receive mercury vapour in heat exchange relation with the steam, the vapour being received from a heating liquid circuit, represented by numeral 3, The heating circuit includes a mercury evaporator 1, conduit means 6 for passing the mercury vapour from the evaporator to the heat exchangers, and further conduit means for returning condensed mercury from the heat exchangers to the evaporator.

There are two sources leading to diificulties in control with reheat stages. Firstly, precautions must be taken to prevent steam stored in the reheater from continuing to drive the turbine in the case of an emergency output; secondly, it must be ensured that the heating surfaces of the reheater in the boiler firing chamber do not become overheated. The usual solution is to provide quick-acting interceptor valves between reheater and turbine and to maintain a steam circulation in the reheater even after cut-out. This solution entails considerable losses of energy due to the substantial pressure drop across the closing organs, while at the same time it is diflicult ot ensure, in the case of several reheat stages, adequate and reliable closure of these valves. An additional measure for preventing the reheater surfaces from overheating may consist in installing them at a relatively cool place within the boiler. The disadvantage of this solution is that the hottest working medium has to pick up heat from the already cooled down flue gases, which increases the size of the heating surfaces to be installed.

In an attempt to overcome these difficulties according to a further feature of this invention, one or more emergency condensers 9 are provided in branches 7 in parallel with the mercury vapour flow coming from the evaporator. The emergency condenser 9 is installed above the mercury condensers 5 of the reheat stages, its top being joined to the mercury vapour duct, while the mercury condensed leaves this condenser at its bottom. Under normal conditions this condenser is shut off, and it only enters into operation in the case of an emergency cut-out. Under normal conditions valve 10 is closed, and as a result the emergency condenser 9 fills up with liquid mercury which prevents further condensation. The emergency condenser being installed at a higher level than the normal condensers, on opening the valve 10 in the case of emergency shutdown the cold condensed mercury falls into the condensing space of the heat exchangers 5 and starts evaporating there, so that steam still flowing over from preceding turbine casings will be cooled instead of being heated, which accelerates the stopping of the turbine. At the same time the emergency condenser described also provides cooling during the shutdown process to the evaporator surfaces in the boiler, since having emptied its mercury filling and again working as a condenser, it will condense any mercury vapour still produced in the boiler; consequently the mercury evaporating surfaces can be installed in hotter fiues of the boiler than otherwise.

Since the steam filling of compact condensers is minimal and according to the foregoing reheatting stops very quickly if necessary, there is no need for interceptor valves, which is a substantial advantage from the point of view of control and reliability in service. But the decisive advantage is the elimination of the substantial pressure drop associated with such valves.

The emergency shutdown equipment can be further refined by devising the cooling side of the emergency condensers in such a manner that condensation is effected by the evaporation of a cooling liquid such as water. For this purpose a liquid separating and storing vessel (boiler drum) 16 of suitable capacity is installed above and joined to the evaporating surfaces 12 of the emergency condenser 9. It is thus ensured that as soon as the valve 10 opens, i.e. as soon as mercury vapour starts flowing into the emergency condenser, evaporation at the surfaces 12 also starts and is maintained by the natural circulation ensuing, This cools the emergency condenser.

From the steam space of the evaporating liquid the steam must be allowed to escape through the relief valve 15 in order to maintain the pressure and consequently the temperature of the cooling medium at a given value.

If cooling of the emergency condenser is realized as described, its cooling will start automatically, without any additional measure, even with a total electric power failure, as soon as mercury vapour enters the emergency condenser, i.e. as soon as cooling is required. Further, if a sufiicient quantity of cooling liquid is stored in the equipment, cooling will also continue as long as it is required, i.e. until heat is produced in the boiler.

We claim:

1. A steam turbine interstage steam reheating system system comprising a plurality of steam reheating stages each including a heat exchanger between successive tur bine stages, means for passing a heating liquid vapour through said heat exchanger, means for passing turbine interstage steam through each said heat exchanger in heat transfer relation with said vapour to eifect condensation of said vapour and reheating of said steam, and a heating liquid circuit having an evaporator for vaporizing the heating liquid and conduit means arranged in parallel for passing vapour from said evaporator through each said heat exchangers and for passing condensed liquid from said heat exchangers back to said evaporator.

2. The system of claim 1 having at least three heat exchangers arranged in parallel in said liquid circuit to provide at least three steam reheating stages.

3. The system of claim Z wherein each heat exchanger is arranged in steam ducting connecting a pair of succeeding turbine stages.

4. The system of claim 1 wherein the heat exchanger includes steam-side fins, the interfin spacing being not greater than 0.5 mm.

5. The system of claim 4 wherein the heat exchanger is arranged in steam ducting connecting a pair of successive turbine stages.

6. The system of claim 1, said heating liquid circuit including first conduit means for passing vapour from the evaporator through each heat exchanger, steam duct means for passing interstage steam through said heat exchangers in heat transfer relation with said vapour, second conduit means for returning condensed liquid from said heat exchangers to said evaporator, an auxiliary liquid condenser at a higher level than said heat exchangers, third conduit means for passing liquid vapour from said evaporator to said auxiliary condenser, and a valve controlled outlet for passing condensate from said auxiliary condenser to said heat exchangers to mix with vapour therein.

7. The system of claim 6 having an auxiliary liquid circuit comprising a storage vessel and conduit means for passing auxiliary liquid from said storage vessel through said auxiliary condenser in heat transfer relation with the heating liquid and thence back to the storage vessel.

References Cited UNITED STATES PATENTS 1,941,480 1/1934 Lucke 12232. 2,865,827 12/1958 Dwyer 6073 X 3,067,592 12/1962 McFarlan 146 X 3,138,199 6/1964 Bell 122-32 X FOREIGN PATENTS 318,983 3/ 1957 Switzerland.

CARROLL B. DORITY, 111., Primary Examiner US. Cl. X.R. 6095 

